The present invention relates generally to the field of material science and engineering. More particularly, the present invention relates to methods for the preparation of dense bulk nanostructured functional oxide materials having a crystallite size of less than about 30 nm.
Nanocrystalline materials have received a widespread attention in the past few years due to their unique combination of physical, chemical, and mechanical properties (e.g., see, Karch, J., Birringer, R. & Gleiter, H, Ceramics ductile at low temperature. Nature 330, 556-558 (1987); McFadden, S. X., Mishra, R. S., Valiev, R. Z., Zhilyaev, A. P. & Mukherjee, A. K. Low-temperature superplasticity in nanostructured nickel and metal alloys. Nature 398 684-686 (1999); Gleiter, H. Nanostructured materials: basic concepts and microstructure. Acta Mater. 48, 1-29 (2000); Moriarty, P. Nanostructured materials. Rep. Prog. Phys. 64, 297-381 (2001); Schoonman, Nanostructured materials in solid state ionics. J. Solid State Ionics 135, 5-19 (2000); Schoonman, Nanoionics. J. Solid State Ionics 157, 319-326 (2003); Cain, M. & Morrell, R. Nanostructured ceramics: a review of their potential. Appl. Organometal. Chem. 15, 321-330 (2001); Yan, D. S., Qiu, H. B., Zheng, Y. S., Gao, L. Bulk nanostructured oxide materials and the superplastic behavior under tensile fatigue at ambient environment. Nanostructured Materials 9, 441-450 (1997); and Mayo, M. J. Processing of nanocrystalline ceramics from ultrafine particles. International Materials Reviews 41, 85-115 (1996)).
Although significant accomplishments have been made in the area of the synthesis of nanometric powders and clusters (e.g., see Yitai, Q. Chemical preparation and characterization of nanocrystalline materials. Handbook of Nanostructured Materials and Nanotechnology 1, 423-480 (2000); Dovy, A. Polyacrylamide gel: an efficient tool for easy synthesis of multicomponent oxide precursors of ceramics and glasses. Int. J. Inorg. Chem. 3, 699-707 (2001); Huang, K. & Goodenough, J. B. Wet Chemical Synthesis of Sr- and Mg-Doped LaGaO3, a Perovskite-Type Oxide-Ion Conductor. J. Sold. State Chem. 136 274-283 (1998); Aruna, S. T., Muthuraman, M., Patil, K. C. Combustion synthesis and properties of strontium substituted lanthanum manganites La1-xSrxMnO3 (0≦x≦0.3). J. Mater. Chem. 7, 2499-2503 (1997); and Grigorieva, T. F., Barinova, A. P., Ivanov, E. Yu. & Boldyrev, V. V. J. Metastable and Nanocrystalline Mat. 15-16, 553-556 (2003)), the goal of synthesizing fully dense bulk material with grain size below 50 nm remains largely unachieved. This is especially true in the case of ceramic materials. In ceramic materials, the synthesis of bulk nanostructured materials relies mainly on the densification of nanopowders, since alternative routes, such as controlled crystallization of bulk amorphous precursor, have found limited application for these materials (e.g., see Rosenflanz, A., Frey, M., Endres, B., Anderson, T., Richards, E.& Schardt, C. Nature 430 761-764 (2004)). As used herein nanopowders refer to materials in powder form having a grain size that is smaller than about 50 nm. The high temperatures required to fully densify ceramic powders result in large grain sizes due to Oswald ripening (e.g., see Cameron, C. P. & Raj Grain growth transition during sintering of colloidally prepared alumina powder compact. J. Am. Ceram. Soc. 71, 1031-1035 (1988)).
To overcome this difficulty, unconventional sintering and densification techniques have been proposed for the densification of nanometric ceramic powders. These include, for example, very high pressure-low temperature densification (e.g., see Liao, S. C., Chen, Y.-J., Kear, B. H. & Mayo, W. E. High pressure/low temperature sintering of nanocrystalline alumina. Nanostruct. Mater. 10, 1063-1079 (1998); and Liao, S. C., Mayo, W. E. & Pae, K. D. Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2. Acta Mater. 45 4027-4040 (1997)), shock densification (e.g., see Jin, Z. Q., Rockett, C., Liu, J. P., Hokamoto, K., Thadhani, N. N., Shock compaction of bulk nanocomposite magnetic materials, Materials Science Forum 465-466, 93-100 (2004)), and magnetic pulsed compaction (e.g., see Ivanov, V., Paranin, S., Khrustov, V., Medvedev, A., Shtol'ts, A., Key Engineering Materials 206-213, 377-380 (2002)).
However, while some success was attained by these methods, the results fall short of the ideal goal of having high relative densities (e.g., greater than 95%) and a grain size below 30 nm (e.g., see Tschöpe, A., Sommer, E. & Birringer, R. Grain size-dependent electrical conductivity of polycrystalline cerium oxide. I. Experiment. Solid State Ionic 139, 255-265 (2001); and Mondal, P. & Hahn, H. Ber. Bunsenges. Phys. Chem. 101, 1765-1766 (1997)). The goal is even more elusive when dense materials with very small grain size (e.g., about 10 nm) are desired. The range of grain size near this value is particularly important since significant variations in bulk physical properties are expected when the grain size approaches this limit (e.g., see Maier, J. Point-defect thermodynamics and size effects. Solid State Ionics 131 13-22 (2000)). At approximately this value, half of the atoms belong to the grain boundary region and thus contribute in a different way to the overall property of the material.
Thus far it has not been possible to prepare dense bulk nanostructured material in general and functional oxides in particular with a fine crystallite size of less than about 30 nm.